This invention relates to a reference point return method and, more particularly, to a reference point return method well-suited for use in performing a reference point return operation in a numerically controlled machine tool employing a digital servo-circuit for digitally generating a velocity command to transport a movable element.
In a numerically controlled machine tool, it is required that a movable element be returned to a predetermined reference point, thereby establishing coincidence between the machine position and a present position in a numerical controller before numerical control starts.
FIG. 1 is a block diagram of a section for practicing a conventional reference point return method, and FIG. 2 is the associated timechart.
If a move command is provided as NC data by an NC tape 10 when a reference point return command ZRN is "0" (i.e. when a reference point return mode is not in effect), a numerical controller 11 calculates, at every predetermined time, a minute traveling distance in the direction of each axis along which a movable element is to be transported during the predetermined time, and inputs the minute traveling distance to a pulse distributor for each axis for each predetermined time until the destination is reached. On the basis of the minute traveling distance inputted thereto, the pulse distributor 12 performs a pulse distribution calculation to generate command pulses P.sub.c. An error counter 13 comprising a reversible counter counts the command pulses P.sub.c. A DA converter, not shown, inputs a voltage proportional to the count to a velocity controller 14 as a velocity command, thereby rotating a servometer 15 to transport a movable element such as a tool or table, not shown.
A rotary encoder 16 is rotated by the rotation of the servometer. The rotary encoder 16 is adapted to generate a single feedback pulse P.sub.F whenever it rotates by a predetermined amount and to generate a one-revolution signal RTS whenever it makes one full revolution. Accordingly, the amount of rotation of the servometer 16 is detected by the rotary encoder 15 and is inputted to the error counter 13 as the feedback pulses P.sub.F, thereby diminishing the contents of the counter in the zero direction. When a steady state prevails, the data (error) in the error counter 13 becomes substantially constant and the servometer 15 rotates at a substantially constant velocity. When the command pulses Pc stop arriving and the number of feedback pulses P.sub.F generated is equal to the number of command pulses, the data in the error counter 13 becomes zero and the servometer 15 stops rotating.
When the referenc point return command ZRN is "1" (i.e. when the reference point return mode is in effect), on the other hand, the numerical controller 11 calculates a minute traveling distance every predetermined time along each axis for transporting the movable element at a rapid-traverse velocity, and inputs these traveling distances to the pulse distributor 12, whereby the movable element is transported just as described above. It should be noted that the actual velocity V.sub.a at this time gradually rises to the rapid-traverse velocity V.sub.R, as shown in FIG. 2.
A gate 17 is open at the start of reference point return. Therefore, the command pulses P.sub.c and the feedback pulses P.sub.F are inputted to a reference counter 18, just as to the error counter 13. The reference counter 18 is comprises a reversible counter and has a capcity N (equivalent to the number of feedback pulses P.sub.F generated during one revolution of the rotary encoder). The contents REF of the reference counter 18 agree with the contents (error) E of the error counter 13.
When the actual velocity V.sub.a exceeds a predetermined first reference velocity VR, the controller 11 generates a control signal D01. After the control signal D01 is generated, a gate control circuit 19 closes the gate 17 in response to generation of the first one-revolution signal RTS. Thereafter, the reference counter 18 counts up only the command pulses P.sub.c, so that the contact REF thereof varies as shown in FIG. 2. A servo delay (=E) at time T.sub.0 is set in the reference counter 18, after which the command pulses P.sub.c are counted. Therefore, the content REF can be regarded as a commanded position reset whenever the capacity N is reached. The actual machine position is as indicated by the one-dot chain line in FIG. 2.
When the movable element travels and depresses a deceleration limit switch provided in the vicinity of the reference point, a deceleration signal DEC assumes a low level (="0"). As a result, the numerical controller 11 executes processing in such a manner that the traveling velocity of the movable element is slowed down to a second reference velocity VR2 (at time T.sub.1), after which the movable element is moved at the velocity VR2.
As the movable element travels further at the low velocity and the deceleration limit switch is restored (time T.sub.2), the numerical controller 11 generates the gate signal D02.
When a predetermined command pulse P.sub.c is subsequently generated, the movable element travels further and the counted value REF in the reference counter 18 becomes zero. As a result, the signal ZR becomes "1" (T.sub.3), whereupon the gate 20 closes and the command pulses P.sub.c are no longer delivered.
Thereafter, the count in error counter 13 gradually diminishes, as a result of which the rotational velocity of the servometer 15 decreases. The count E in error counter 13 finally comes to rest at zero when the one-revolution signal RTS is generated.
In accordance with this conventional reference point return method, the return to the reference point can be performed accurately independently of the amount of delay. In addition, if a one-revolution position is referred to as a grid point, the movable element can be reference point-returned to the first grid point after the deceleration limit switch is restored. Morever, by presetting a numerical value M in the reference counter 18, a position displaced M pulses from the grid point position can be made the reference point-return position. Thus, the conventional method is a useful one.
There has been a recent trend toward digital control of servomotors. With such a digital servo, a traveling distance .DELTA.R.sub.n to be traveled along each axis every predetermined time .DELTA.T (e.g. 2 msec) is calculated by a numerical controller. A value obtained by multiplying the traveling distance .DELTA.R.sub.n by a predetermined gain is inputted to a digital servo-circuit every predetermined time .DELTA.T, and the digital servo-circuit performs a calculation in accordance with the following equation every .DELTA.T: EQU E+.sub..DELTA. R.sub.n -.sub..DELTA. P.sub.n .fwdarw.
(where .DELTA.P.sub.n is the actual traveling distance every .DELTA.T and E a cumulative error whose initial value is zero) and executes pulse-width modulation in accordance with the size of the error E, thereby controlling the rotational velocity of the servomotor.
In other words, in the case of a digital servo, the pulse distributor 12 which generates the serial pulses is not provided, and the error counter is substituted by a RAM in the numerical controller 11.
A problem which results is that the conventional reference point return method cannot be applied to reference point return of a numerically controlled machine employing such a digital servo.